I was investigating an issue with seabios >= 1.10 which stopped working
for nested KVM on Hyper-V. The problem appears to be in
handle_ept_violation() function: when we do fast mmio we need to skip
the instruction so we do kvm_skip_emulated_instruction(). This, however,
depends on VM_EXIT_INSTRUCTION_LEN field being set correctly in VMCS.
However, this is not the case.
Intel's manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set when
EPT MISCONFIG occurs. While on real hardware it was observed to be set,
some hypervisors follow the spec and don't set it; we end up advancing
IP with some random value.
I checked with Microsoft and they confirmed they don't fill
VM_EXIT_INSTRUCTION_LEN on EPT MISCONFIG.
Fix the issue by doing instruction skip through emulator when running
nested.
Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
Suggested-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
v1 -> v2:
inlay X86_FEATURE_HYPERVISOR case with EMULTYPE_SKIP optimization
[Paolo Bonzini, Radim Krčmář]
---
arch/x86/kvm/vmx.c | 16 +++++++++++++++-
arch/x86/kvm/x86.c | 3 ++-
2 files changed, 17 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index c829d89e2e63..e105b439c372 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6563,7 +6563,21 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
if (!is_guest_mode(vcpu) &&
!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
trace_kvm_fast_mmio(gpa);
- return kvm_skip_emulated_instruction(vcpu);
+ /*
+ * Doing kvm_skip_emulated_instruction() depends on undefined
+ * behavior: Intel's manual doesn't mandate
+ * VM_EXIT_INSTRUCTION_LEN to be set in VMCS when EPT MISCONFIG
+ * occurs and while on real hardware it was observed to be set,
+ * other hypervisors (namely Hyper-V) don't set it, we end up
+ * advancing IP with some random value. Disable fast mmio when
+ * running nested and keep it for real hardware in hope that
+ * VM_EXIT_INSTRUCTION_LEN will always be set correctly.
+ */
+ if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
+ return kvm_skip_emulated_instruction(vcpu);
+ else
+ return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP,
+ NULL, 0) == EMULATE_DONE;
}
ret = kvm_mmu_page_fault(vcpu, gpa, PFERR_RSVD_MASK, NULL, 0);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1cec2c62a0b0..930aba87a723 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5703,7 +5703,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
* handle watchpoints yet, those would be handled in
* the emulate_ops.
*/
- if (kvm_vcpu_check_breakpoint(vcpu, &r))
+ if (!(emulation_type & EMULTYPE_SKIP) &&
+ kvm_vcpu_check_breakpoint(vcpu, &r))
return r;
ctxt->interruptibility = 0;
--
2.14.3
